An LED is a useful device for many display and communication applications. Typically, an LED is a p-n junction made of a direct bandgap semiconductor. If the LED is forward-biased, positive carriers (i.e. holes) are injected into the n-side of the p-n junction, and negative carriers (i.e. electrons) are injected into the p-side of the p-n junction. The injected carriers recombine, causing photons to be released. Also, the wavelength of light provided by a forward-biased LED is a function of the bandgap voltage of the semiconductor. Generally, direct bandgap semiconductors are used for LEDs because radiative carrier recombination typically dominates in direct bandgap semiconductors, leading to light emission. Conversely, in indirect bandgap materials, most of the carrier recombination paths are nonradiative, generating heat instead of light.
According to a first approach, an LED may be driven with a DC voltage, and the DC voltage may be adjusted to adjust the brightness of the LED. According to a second approach, an LED may be driven with a high peak current having a low duty cycle, and the duty cycle of the current may be adjusted to adjust the brightness of the LED. The second approach consumes less power at a given level of brightness than the first approach.